The present invention relates to a process for producing a water-absorbent resin. In detail, this invention relates to a process for producing a water absorbent resin, which has high absorption capacity and in which water-soluble ingredients are contained in only a small amount.
The water-absorbent resin has been so far used for various kinds of water-absorbent materials such as a paper diaper (or a disposable diaper), physiological articles, a soil water-retentive agent. As a water-absorbent resin of this type, there have been known a hydrolysis product of a starch-acrylonitrile graft copolymer, a neutralized product of a starch-acrylic acid graft copolymer, a crosslinked product of acrylic acid or an acrylate polymer, a partially crosslinked product of polyethylene oxide, a crosslinked product of carboxymethylcellulose and the like. Why the crosslinked polymer is used as a water-absorbent resin is to prevent conversion into a sol by its infinite swelling in absorbing water followed by swelling.
In the above-described crosslinked polymers, a crosslinked polymer obtained by copolymerizing a water-soluble monoethylenic unsaturated monomer such as acrylic acid in the presence of a crosslinking agent is of low cost as raw materials, superior in water-absorbent properties, and needs not to care about decomposition, so that it is a main current of the water-absorbent resin.
Preferable crosslinking agents so far known are N,N'-methylenebis(meth)acrylamide, N-methylol(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate or (poly)propylene glycol di(meth)acrylate (refer to U.S. Pat. No. 4,351,922), glycerol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, triallylamine, triallyl cyanurate, triallyl phosphate, di(2-acryLoyloxyethyl) acid phosphate (refer to JP-B-62-56888), glycidyl (meth)acrylate, polyethylene glycol diglycidyl ether (refer to U.S. Pat. No. 4,351,922), glycerol and the like.
The real situation of conventional water-absorbent resins produced by using the aforementioned crosslinking agents is that, as the absorption capacity becomes higher, the amount of water-soluble ingredients becomes larger. If such a water-absorbent resin having a large amount of water-soluble ingredients is used for a diaper and the like during a long time, there is a problem that a slime generates on the diaper surface, and another problem that the liquid permeability becomes bad and thereby, the water-absorbent capacity and liquid-dispersing character decrease, so that a leak easily occurs. Conversely, if reduction of the water-soluble ingredients in a water-absorbent resin is intended by increase of the amount of a crosslinking agent, the water-absorbent capacity decreases and an usable range of the water-absorbent resin is limited.